Sensors and Actuators A 219 (2014) 94–99

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Sensors and Actuators A: Physicaljournal homepage: www.elsevier.com/locate/sna

Refractive index and strain sensing using inline Mach–Zehnderinterferometer comprising perﬂuorinated graded-index plastic opticalﬁber
A.A. Jasim a,b , N. Hayashi c , S.W. Harun a,b,∗ , H. Ahmad b , R. Penny b , Y. Mizuno c ,K. Nakamura caDepartment of Electrical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia
Photonics Research Center, University of Malaya, Kuala Lumpur 50603, MalaysiacPrecision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japanba r t i c l e

i n f oArticle history:
Received 18 April 2014
Received in revised form 23 July 2014
Accepted 23 July 2014
Available online 19 August 2014
Keywords:
Tapered ﬁbers
Plastic optical ﬁbers
Heat-and-pull technique
Inline Mach–Zehnder interferometry
Refractive-index sensing
Strain sensinga b s t r a c t
This paper details the fabrication and performance evaluation of an inline Mach–Zehnder interferometer (MZI) consisting of a perﬂuorinated graded-index plastic optical ﬁber fabricated via a heat-and-pull
tapering technique. This MZI exhibits interference fringes that are sufﬁciently sharp for usage as
refractive-index (RI) and strain sensors. These interference fringes shift linearly with the RI of the surrounding medium, with a proportionality constant of 3.44 nm/RI unit. The measured strain sensitivity of
0.2 pm/␮␧ suggests that the MZI may be viable for strain-sensing applications.
© 2014 Elsevier B.V. All rights reserved.

1. Introduction
Fiber inline interferometric sensors (FIISs) have drawn greatinterest in recent years thanks to their compactness, high sensitivity, ease of fabrication, and feasibility with a variety ofmeasurands such as temperature, strain, and refractive index (RI).
Several FIIS conﬁgurations have already been developed, includingMach–Zehnder interferometers (MZIs) based on photonic crystal
ﬁbers (PCFs) [1,2] or long-period ﬁber gratings (LPFGs) [3], andFabry–Perot interferometers based on micro-holes [4] or hollowcore PCFs [5,6]. Such FIISs have the disadvantage of requiringhigh-cost specialty ﬁbers and/or large dimensions. Consequently,
FIISs utilizing tapered optical ﬁbers, with the associated beneﬁts
of cost efﬁciency and smaller size [7], have attracted considerable interest. Taper-based FIISs, until now, have been exclusively
fabricated using glass optical ﬁbers that are too fragile to withstand strains beyond roughly 3%. Replacing the glass optical ﬁbers
with plastic (or polymer) optical ﬁbers (POFs) [8–14] is a promising∗ Corresponding author at: Department of Electrical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia. Tel.: +60 379674290.
E-mail address: swharun@gmail.com (S.W. Harun).
http://dx.doi.org/10.1016/j.sna.2014.07.018
0924-4247/© 2014 Elsevier B.V. All rights reserved.

approach to overcome this limitation, offering advantages including ease of handling, high mechanical strength and load resistance,
extremely high ﬂexibility, and tolerance of strains in excess of 50%.
Although some sensing applications of tapered POFs have alreadybeen demonstrated [15–19], no reports on the fabrication and characterization of POF-based inline interferometric RI/strain sensors
have been published to the best of the authors’ knowledge.
This paper reports on the ﬁrst tests of an inline MZI made froma perﬂuorinated graded-index (GI-) POF, tapered by a heat-andpull technique, along with an evaluation of its potential application
as an RI and strain sensor. While the concept of using an MZI for
measuring phase shift in the POF as a function of nominal strain
has been reported previously [20], the assembly required a complex setup involving several components for conﬁguring the MZI.
In contrast, the experiment described in this paper uses a very
short length of (GI-) POF, measuring approximately 10 mm, to fabricate an inline MZI of simple design that is suitable for use as an
RI and strain sensor. Experimental results show that the interference fringes shift in proportion to the RI of a surrounding medium
with a proportionality constant of 3.44 nm/RI unit and a linear correlation of 98%. In addition, the MZI sensitivity was measured at
0.2 pm/␮␧, indicating that the device may be regarded as a strain
sensor.

A.A. Jasim et al. / Sensors and Actuators A 219 (2014) 94–99

95

Fig. 1. Schematic diagram of the inline MZI.

2. Principles of inline MZIs
A diagram illustrating the principles of the inline MZI operation is shown in Fig. 1, and the intensity transfer function can be
expressed generally by a two-beam optical interference equation:

I = I1 + I2 + 2

I1 I2 cos(ϕ),(1)

where I is the intensity of the interference signal, and I1 and I2
are the intensities of the light propagating in the ﬁber core andcladding, respectively. The phase difference between the core andcladding modes is ϕ, which is approximately equal toϕ= 2(n )L eff eff


,

(2)

eff
where neff is deﬁned as (neffc − ncl,m ), i.e. the difference between
the effective RIs of the core mode and the m-th order cladding mode
respectively, Leff is the effective length of the interferometer, and is the optical wavelength. Eq. (2) indicates that the phase difference between the core and cladding modes is directly dependent
on the difference in their effective RI as well as the effective lengthof the interferometer. Input light in an inline MZI is generally split
into guided and unguided modes at the interference region; guided
modes continue to travel in the core, while unguided modes extend
through the cladding–air interface, thus allowing the evanescentﬁeld to react with the ambient medium. An interference signal
occurs when the two modes are recombined in the second tapered
region. This reaches a minimum intensity (I = Imin ) when the phasedifference between the core and cladding modes satisﬁes the condition ϕ = (2k + 1), where k is an integer. The attenuation peakwavelength v of the kth order can then be expressed as

v =

eff
2(neffc − ncl,m )Leff(2k + 1).

(3)

In general, the cladding modes at the tapered sections are readily
affected by the RI of the surrounding medium, and thus the effective
is highly sensitive to RI variations in
RI of the cladding modes neffcl,mthe surrounding medium. The effective RI of the core modes neffcdoes not vary with the ambient index change, and is thus highlyinvariant. Therefore, the relationship of neffwith respect to v iscl,mgiven as
dv
dneff
cl,m

=

2Leffv,=
eff(2k + 1)(n
− neff
c )

(4)

cl,m

Consequently, it can be asserted that the RI-sensing mechanism of the inline MZI relies on the shifting of the attenuation
peak wavelength v from the variation of ambient RI, whereby achange occurs in effective RI difference neff between the core andcladding modes. The strain-sensing mechanism of the inline MZIrelies on the shift in v from the elongation of the interferometer,
causing a change both in Leff and neff .
3. Fabrication and characterization of inline MZIs
Two typical commercially available POFs are poly(methyl
methacrylate)-based POFs [8,9] and perﬂuorinated GI-POFs[10–14]. The former transmit mainly visible light at around
650 nm, whereas the latter transmit both visible light and telecomwavelength light. An ampliﬁed spontaneous emission (ASE) source
at 1.55 ␮m was used in this experiment, and so the inline MZI
was fabricated from a perﬂuorinated GI-POF. The core and cladding
layers of the GI-POF consisted of doped and undoped amorphous
perﬂuorinated polymer (polyperﬂuorobutenylvinyl ether), respectively. The RI at the center of the core was 1.356, whereas that of thecladding layer was 1.342. The numerical aperture (NA) was 0.185
and the core/cladding diameters were 50/100 ␮m. This GI-POF
has a 750 ␮m diameter polycarbonate reinforcement over-cladding
layer, which reduces microbending loss and increases load-bearing
capability. Propagation loss at 1.55 ␮m wavelength was approximately 250 dB/km.
Fig. 2 shows a schematic for fabricating a POF-based inline MZI,
where a meter-long POF was afﬁxed using two linear translation
stages. A solder gun set at 110 ◦ C was placed 5 cm below the POF,
acting as a heat source. Both ends of the POF were butt-coupled[12] to standard silica single-mode ﬁbers (SMFs), i.e., the ends ofFig. 2. Schematic for fabricating an inline MZI.

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Fig. 3. An optical microscope image of the fabricated inline MZI.

-15

-20

-25

-25

-35
-30
-45
-35
-55

-40

Input ASE

-65

Transmission (dB)

Output Power (dBm)the SMFs ﬁtted with ‘FC’ connectors were attached to both ends of
the POF ﬁtted with ‘SC’ connectors via ‘FC/SC’ adaptors. This conﬁguration resulted in precisely aligned cores predominantly exciting
the fundamental propagation mode. One SMF was connected toa broadband ASE source and the other SMF was connected to an
optical spectrum analyzer (OSA). A portion of the POF was heatedto a softening temperature then stretched until the over-cladding
diameter was reduced to 315 ␮m. The solder gun was then moved
to one side of the POF, allowing another POF section to be tapered
further until the over-cladding diameter decreased to 237 ␮m. An
optical microscope image of the POF-based inline MZI fabricatedby this process is shown in Fig. 3, where the total tapered lengthis approximately 10 mm. Fig. 4 shows the output spectrum of theinline MZI alongside the incident ASE spectrum. The dashed line
in Fig. 4 represents the transmission spectrum normalized to the
input ASE spectrum, which contributes to an improved visibility.
The ∼35-dB insertion loss of the inline MZI results predominantly
from inefﬁcient coupling at the interfaces between the silica SMF
and the POF.

Output ASE from (MZI)
Normalized
-75
1521

-45
1531

1541

1551

1561

1571

Wavelength (nm)Fig. 4. Spectra from the incident ASE, and the inline MZI output.

4. Applications of inline MZI for RI and strain sensing
Fig. 5 shows the RI-sensing experimental setup, where the POFbased inline MZI is placed in a plastic container ﬁlled with air,water, or one of four types of oil. These oils are differentiated bytheir respective RIs, ranging from 1.44 to 1.74. The OSA is used
to measure the change in output spectrum, which is a function of
RI. Fig. 6(a) shows the transmitted interference spectra at around
1537 nm for the six ﬂuids, where the wavelengths of the spectral peaks are red-shifted with increasing RI from 1.00 to 1.74.

Noticeable red-shifting occurs under these conditions because thehigher-order cladding mode makes a signiﬁcant contribution to thelight interference, which is a result of the non-uniform tapering
shape of the inline MZI. Moreover, according to Eq. (4), the shifvaries for different mth order cladding modes.
ting of dv /dneffcl,mA similar phenomenon was observed for the standard silica SMFs
and LPFG-based RI sensors [21,22]. It can be also observed from
Fig. 6(a) that the peak power of the wavelength decreases withincreasing RI. An increase in RI causing this reduction of power can

Fig. 5. Schematic for RI sensing.

A.A. Jasim et al. / Sensors and Actuators A 219 (2014) 94–99
0-51
air 1.00-5

water 1.33

-52

-10

oil 1.44

Output Power (a.u)

Output Power (dBm)97

oil 1.48-53

oil 1.60
oil 1.74
-54

-55

-15

-20

-25
0.67 % strain
1.33%
2.00%
2.67%

-30

-56
1534

-35
1513

1535.51537

1538.5

1540

1518

1.00%
1.67%
2.33%
3.00%
1523

1528

1533

1538

Wavelength (nm)1541.5(a)Wavelength (nm)

(a)
1531

Wavelength (nm)Wavelength (nm)1538.51537.51529
r = 0.98
1527

Slope: 3.44 nm/RIU
1525

1536.5

0.5

1.1

1.7

2.3

2.9

3.5

Strain (%)(b)
1535.5
1

1.2

1.4

1.6

1.8

Refractive Index

Fig. 7. (a) Transmitted interference spectra at around 1528 nm with different strains
applied. (b) Dip wavelength correlation with strain.

(b)
Fig. 6. (a) Transmitted interference spectra at around 1537 nm with six ﬂuids used.
(b) Peak wavelength correlation with RI.

be attributed to most of the cladding modes’ power being absorbed
in the surrounding high-RI medium. The peak wavelength is plotted as a function of RI as shown in Fig. 6(b). This dependence showsa strong linear ﬁt, with a correlation coefﬁcient of r > 0.98 and a sensitivity of 3.44 nm/RI unit (RIU). This value is much lower than thesensitivities of standard silica SMF-, PCF-, and LPG-based RI sensors, which are on the order of hundreds of nm/RIU [22–24]. This
reduced sensitivity occurs mainly because the diameter of the fabricated POF-based inline MZI is much greater than the diametersof the other ﬁbers. Enhancing the RI sensitivity will require a thinner POF adopted in the fabrication of the inline MZI, as well as anincrease in interferometer length. Such measures should increase
the cladding modes’ propagation into the ambient medium, andbeing affected signiﬁcantly with the change of thethus lead to neffcl,mambient RI. In practice, this inline MZI sensor is capable of monitoring an aqueous medium with refractive index ranging from 1.33
to 1.74.
The strain dependence of the transmitted interference spectra
near 1528 nm at room temperature was also investigated. One endof the inline MZI was ﬁxed on a stationary platform and the otherend was set on a translation stage with a calibrated 1-␮m screw
clamp having a resolution of 2 ␮m/step. The gauge length between
two stages was 60 mm, and the POF-based inline MZI was held tautacross this region. Strains of up to 3%, corresponding to 1.8 mmof the gauge length, were applied to the POF-based inline MZI.
Fig. 7(a) shows the resulting spectral variation for applied strains of
0.67–3.00%, where the spectral dip experiences an increased redshift with additional strain. This correlation arises because, in thissetup, the interferometer elongation has a greater impact on wavelength than changes in neff , resulting in a phase shift toward
longer wavelength. Strain dependence on the wavelength at the
spectral dip also shows a strong linear ﬁt with a correlation ofr > 0.98 and a sensitivity of 0.2 pm/␮␧, as shown in Fig. 7(b).
The inline MZI strain sensitivity is lower than the 1.2 pm/␮␧result from strain applied to ﬁber Bragg gratings fabricated inuniform-waist ﬁber tapers [25]. This relative drop in sensitivity
can be explained by taking into account the non-uniform, tapered
shape of the POF-based inline MZI; the two tapered regions experience an unequal strain due to local mechanical resistance, andthis imbalance results in a reduction of sensitivity. Nevertheless,the strain sensitivity of the POF-based inline MZI remains comparable with that of the non-tapered LPFG all-solid photonic bandgap
ﬁber, where the sensitivity difference observed is 0.792 pm/␮␧
[26]. Still, the fabricated inline MZI retains the relative advantages of design simplicity and lower cost. Moreover, POF-based
sensors retain the key advantage of high ﬂexibility, showing amuch greater potential for large-strain sensing than that of glassﬁber-based sensors. Their thermal stability was also sufﬁciently
high for practical applications. Issues with relatively low sensitivity
and large loss could be mitigated: POFs with different structures
such as smaller core diameters might improve the sensitivity,and propagation loss could be lowered to ∼10 dB/km by using a

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shorter source wavelength (1000–1100 nm or 1230 nm) [11]. Thus,
overall, the POF-based inline MZI is suitable for applications as an
RI and strain sensor.
5. Conclusion
Combined RI and strain sensing is demonstrated for the ﬁrst
time with an inline MZI made from a perﬂuorinated GI-POF tapered
by a heat-and-pull technique. Interference fringes of the MZI are
observed to shift toward longer wavelength when the RI of thesurrounding medium increases from 1.00 to 1.74. The RI sensitivity is 3.44 nm/RIU. Strain-sensing capability with a sensitivity of
0.2 pm/␮␧ was also conﬁrmed. The authors believe these results
will serve as an important guideline toward the development of
more reﬁned RI and strain sensors derived from POF-based inline
MZIs.
Acknowledgement
The authors acknowledge the ﬁnancial support from the University of Malaya and the Ministry of Higher Education (MOHE) under
grants, UM.C/625/1/HIR/MOHE/SCI/29, RU002/2013 and PG1392012B. This work was also partially supported by Grants-in-Aid
for Young Scientists (A) (no. 25709032) and for Challenging
Exploratory Research (no. 26630180) from the Japan Society for
the Promotion of Science (JSPS), and research grants from the General Sekiyu Foundation, the Iwatani Naoji Foundation, and the SCAT
Foundation. N.H. acknowledges a Grant-in-Aid for JSPS Fellows (No.
25007652).

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Biographies

A.A. Jasim received a B.Sc. degree in Electrical Engineeringfrom the University of Technology, Baghdad, Iraq, in 2007,
and M.E. and Ph.D. degrees in Photonic and Fiber Optic
Technology from the University of Malaya, Malaysia, in2010 and 2014, respectively. He is currently a Research
Fellow at the Photonics Research Centre, University of
Malaya. His research interest is in optical micro/nanoﬁber
photonic devices and their applications.

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N. Hayashi received his received a B.E. degree in Advanced
Production from the Gunma National College Technology,
Japan, in 2011, and an M.E. degree in Electronic Engineering from the Tokyo Institute of Technology, Japan, in 2013.
He is currently studying polymer optics for his Dr. Eng.
degree at the Tokyo Institute of Technology.

S.W. Harun received a B.E. degree in Electrical and Electronics System Engineering from the Nagaoka Universityof Technology, Japan in 1996, and M.Sc. and Ph.D. degrees
in Photonics from the University of Malaya in 2001 and
2004, respectively. Currently, he is a full professor at theFaculty of Engineering, University of Malaya. His researchinterests include ﬁber-optic active and passive devices.

H. Ahmad received a Ph.D. degree in laser technology fromthe University of Swansea (UK) in 1983. He is a full professor at the Department of Physics and director of the
Photonics Research Centre, University of Malaya. He is alsoa fellow member of the Malaysian Academic of Science.

A.A. Jasim et al. / Sensors and Actuators A 219 (2014) 94–99

R. Penny received a Ph.D degree in high energy physics
from the University of Birmingham (UK) in 2004. He iscurrently a research consultant at the Photonics ResearchCentre, University of Malaya.

Y. Mizuno received B.E., M.E., and Dr. Eng. degrees in Electronic Engineering from the University of Tokyo, Japan,
in 2005, 2007, and 2010, respectively. He is currently an
Assistant Professor at Precision and Intelligence Laboratory, Tokyo Institute of Technology, where he is active inﬁber-optic sensing, polymer optics, and ultrasonics.

99

K. Nakamura received B.E., M.E., and Dr. Eng. degrees
from Tokyo Institute of Technology, Japan, in 1987, 1989,
and 1992, respectively. He is currently a Professor at thePrecision and Intelligence Laboratory, Tokyo Institute ofTechnology. His ﬁeld of research is the applications ofultrasonic waves, measurement of vibration and soundusing optical methods, and ﬁber-optic sensing.